Xenon is a colorless, heavy, odorless noble gas with nine naturally occurring, stable isotopes. Xenon is naturally present in the Earth's atmosphere at a concentration of 0.09 parts per million by volume. Naturally occurring atmospheric xenon is comprised of approximately 21% of the isotope Xe-131 (Xe-131). Of particular interest to the present invention, Xenon-131 is one of xenon's stable isotopes, while Xe-131m is a metastable, radioactive isotope which decays to the ground state of Xe-131 through a 163.9 keV isomeric transition. Xenon-131m has a half-life of approximately 11.8 days.
Xe-131 is a product of multiple processes. In one process, Xe-131 is produced from the decay of I-131. In another process, Xe-131 is formed as a fission product of both U-235 and Pu-239, and is therefore useful as an indicator of nuclear explosions. Xenon that is a product of fission is commercially available; however, it contains up to five different isotopes of xenon that requires separation through physical means with a mass separator or other similar instrument in order to obtain a pure form of a specific isotope. Commercial fission product xenon cannot be separated via chemical means.
In the past twenty-five years, one xenon isotope—xenon-129 (Xe-129)—has proved to be particularly effective for use in nuclear magnetic resonance (NMR) spectroscopy and in magnetic resonance imaging (MRI) applications. See e.g., Ripmeester, J. A., J. Am. Chem. Soc. (1982), 104, 289 and U.S. Pat. No. 5,617,860 issued to Chupp, et al. With respect to NMR, Xe-129 is useful as a probe of void space in solids. Specifically, Xe-129, with a spin S=1/2, can provide information about the symmetry and structure of internal surfaces because the isotropic chemical shift is sensitive to void size while the anisotropic shift is sensitive to void symmetry.
More recently, Xe-131 has been investigated as a potential NMR probe of void space. While use of Xe-131 in NMR spectroscopy of solid phases has its challenges due to its low natural abundance (21.2% of atmospheric xenon), low resonance frequency (24.6 MHz in a magnetic field of 7.05 T), relatively large quadrupole moment (−0.12×10−24 cm2) and its NMR sensitivity (10% that of 129-Xe), it has proven to be a powerful probe that can provide unique information about a void space. In addition, Xe-131 has a spin S=3/2 and, thus, a quadrupolar moment that can be exploited to provide valuable information that cannot be obtained from Xe-129 NMR.
Thus far, Xe-131 has been used, among other uses, in NMR applications to provide information on relaxation due to fluctuating electric fields, as a contrast agent for microimaging aerogels, to probe the distortion of the atomic electron density by an external magnetic field, and to investigate macroscopic void space through quadrupolar coupling.
In addition, the radioactive isotope Xe-131m has proven useful for energy calibration on gamma counting equipment due to its production of an electron at 164 keV. Consequently, isotopically pure Xe-131m allows for the absolute determination of 164 keV on gamma ray counting equipment.
As discussed above, Xe-131 and Xe-131m can be useful in a number of applications. The previously described applications of Xe-131 are purely exemplary and are not meant to be limiting. One of ordinary skill in the art will appreciate additional uses of Xe-131 and Xe-131m that are not described above.
As additional uses of Xe-131 and Xe-131m become known and are more commonly utilized, it is desirable to have an easy, efficient and safe manner in which to obtain isotopically enriched compounds of Xe-131 and Xe-131m, as well as other isotopes of various elements. Previous methods that have been utilized to separate xenon from other compounds, or produce Xe-131 specifically, are deficient for a number of reasons, including that they are laborious, inefficient and require large amounts of starting material, thus increasing personnel exposure to potentially harmful compounds.
The general mechanism by which Xe-131 and Xe-131m are currently produced is through the beta decay of iodine-131 (I-131), a radioisotope of iodine. Iodine-131, with a half-life of approximately 8 days, decays to Xe-131 (99%) and Xe-131m (1%). Xenon-131 is a stable isotope, while Xe-131m is a metastable state which decays to the ground state of Xe-131 and has a half-life of approximately 11.8 days.
The prior methods for obtaining isotopes of xenon rely on a number of techniques and starting materials, but essentially rely on the decomposition of iodine described above. As a radioisotope of iodine, I-131 decays naturally to Xe-131 through the emission of beta particles (606 keV) and gamma rays (364 keV), as exhibited by the following equation:
      131    53    |                    ⟶                  131          54                    ⁢      Xe        +                                     -          1                0            ⁢      β        +                           0        0            ⁢      Y      
Consequently, in producing Xe-131 and Xe-131m, there is the potential that personnel could be exposed to radiation in the process of producing radioactive Xe-131m and the stable isotope Xe-131.
As reflected in Table 1, the decay of I-131 produces a maximum amount of Xe-131m around approximately 14 days. Thereafter, Xe-131m decays to Xe-131 through the 164 keV transition.
TABLE 1Decay of I-131 and Activity of Xe-131mXe-131mDayI-131 (Ci)(Ci)01.00E+000.00E+0019.17E−015.42E−0428.41E−011.01E−0337.72E−011.41E−0347.08E−011.75E−0356.49E−012.03E−0365.95E−012.27E−0375.46E−012.46E−0385.01E−012.62E−0394.59E−012.74E−03104.21E−012.83E−03113.87E−012.90E−03123.55E−012.95E−03133.25E−012.97E−03142.98E−012.98E−03152.74E−012.97E−03162.51E−012.95E−03172.30E−012.92E−03182.11E−012.88E−03191.94E−012.83E−03201.78E−012.78E−03211.63E−012.72E−03
Xenon can be separated from iodine by a variety of methods. One such method is through the separation of Xe-131m bound to xenon oxides. Chromatographic separation of certain oxygen compounds of xenon and iodine, A. N. Mosevich, N. P. Kuzentsov, Y. G. Gusev, translated from Radiokhimiya, vol. 7, no. 6, pp. 678-687 (November-December 1965). In that method, chemically bound Xe-131m was chromatographically isolated in a pure form with the aid of a XeO3 carrier. However, this method is laborious and leaves behind a substantial amount of unrecovered Xe-131m isotope.
In addition, thin-layer, ion-exchange and paper chromatography, as well as electrophoresis, have been used as methods to separate Xe-131m-labeled XeO3 from iodine compounds. Use of the Method of Thin-Layer Chromatography for the Separation of Xenon Trioxide from Oxygen-Containing Iodine Compounds, I. S. Kirin, V. K. Isupov, Y. K. Gusev, translated from Radiokhimiya, vol. 12, no. 3, pp. 500-505 (May-June 1970). However, this method does not recover a pure form of Xe-131m or Xe-131.
Similarly, one exemplary process used by the present inventor for producing Xe-131m starts with venting and capping a bottle containing I-131 in a basic solution, such as sodium hydroxide. The bottle is placed in a chamber that has an inlet for helium, or other sweep gas, and an outlet to a trap containing charcoal. The sodium hydroxide solution containing I-131 undergoes beta decay to produce Xe-131m in the headspace of the capped bottle. A maximum amount of Xe-131m is produced approximately 14 days after the commencement of trapping the xenon, as shown in Table 1 above. After this time, the bottle is opened in the chamber and helium is introduced into the chamber in order to sweep the Xe-131 onto a charcoal trap. This method isolates Xe-131m, but also recovers amounts of I-131 which require further chromatographic separation.
The stable isotope Xe-131 is obtained in a similar manner by allowing the I-131 to decay for a longer period of time in order to produce Xe-131 that is essentially free of Xe-131m.
However, a number of problems arise that make this method unattractive for maximum recovery of isotopically pure Xe-131m. First, in sweeping the chamber, significant amounts of I-131 are also swept onto the charcoal trap. I-131 must then be separated from Xe-131m or Xe-131 through chromatographic means, which is inefficient at collecting the maximum amount of Xe-131m or Xe-131. For example, approximately one-third of the Xe-131m can be lost during the chromatographic separation. Second, a substantial amount of Xe-131m remains on the charcoal trap unrecovered after heating. Finally, a significant amount of Xe-131m is left behind in the sodium hydroxide solution due to the solubility of xenon in water. Thus, in order to recover an adequate amount of Xe-131m, a substantial amount of the starting material containing I-131 is necessary. For example, more than approximately 100 mCi of I-131 is required in order to recover a measurable amount of Xe-131m. Moreover, this method is not an improvement over prior methods because it still requires the step of separating I-131 from Xe-131m via chromatographic means, which is time consuming and laborious.
The foregoing methods of separation and production of Xe-131 or Xe-131m are labor intensive, complicated and time-consuming due to the requirement that the xenon be separated from iodine via chromatographic or other separation methods. More importantly, significant amounts of Xe-131m or Xe-131 are not recovered using the above methods, thus requiring a substantial amount of starting material in order to recover an adequate amount of the desired xenon isotope. Thus, there is a need to provide an improved simple, safe and efficient process for producing isotopically pure Xe-131m and Xe-131. All references cited herein are incorporated by reference in their entireties.